Safety device for preloaded bolts

ABSTRACT

A safety device used in preloading of bolts for providing the bolts with an internal stress in a mounted condition, wherein a first end of the bolt is secured to a first element and extends through a second element, which clamps to the first element, and where a hydraulic preloading device is temporarily applied to a second end of the bolt for axially stretching the bolt to create the external stress. The safety device incorporates a safety tensile bar detachably fitted to the first end of the bolt, and extending through a central bore in the bolt, and further through the hydraulic preloading device while being held adjacent its opposite end in an expanding plug. The expanding plug is provided in a frictional contact with a sleeve, which guides the plug to create a counter hold for absorbing relieved energy in case of the breaking of the bolt during the preloading operation.

The present invention relates to preloaded bolts and more particularlyto hydraulically preloaded bolts subjected to high preloads, and morespecifically it provides a safety device for such preloaded bolts.

For locking different elements to each other it is well known to usebolt and nut joints. It furthermore is well known to stretch such boltsaxially before the nuts are tightened, whereby the bolt joint will haveinternal stresses preventing the nut from being untightened duringoperation.

Bolt joints of this kind are used, for example, as casing bolts forsecuring the casing of steam and gas turbines to the pressure vessel.Such applications require large preloads and the bolt joints will besubjected to very high tensile stresses particularly during the mountingand preloading operations, and this means that there always is a riskthat the bolts during mounting and preloading will develop crackformation, which in some cases also may result in bolt rupture.

Breaking tests may show that the heavy nut and bolt fractions, whichpart from the rest of the bolt, at such a breakage will be thrownseveral tens of meters up in the air for a vertically positioned bolt.This of course presents a large and serious risk for damages to materialand injuries to personnel.

The purpose of the present invention is to provide a safety device forsuch preloaded bolt joints, by which the risk for such damages andinjuries is substantially reduced, and for accomplishing this the safetydevice according to the invention has been given the features defined inthe accompanying claim 1.

Hereinafter the invention will be further described with reference to anon-limiting embodiment illustrated in the accompanying drawing.

FIG. 1 shows in a longitudinal cross section a bolt joint provided witha safety device according to the invention.

FIGS. 2-4 are enlarged detail views from FIG. 1.

FIG. 1 shows in a longitudinal cross section a fraction of a firstelement 1, e.g. a pressure vessel wall, and in which is provided athreaded blind hole 2, wherein is screwed in the threaded free end of abolt 3. The shank 4 of the bolt 3 extends with clearance through athrough-bore 5 in a second element 6, e.g. a casing for the pressurevessel.

A thrust washer 7 is arranged in an annular recess 8 in the secondelement 6 encircling the through-bore 5 thereof. This thrust washer 7serves as a support for a nut 9 which is tightened on the threaded endof the bolt 3 projecting from the second element 6 and the thrust washer7. In the free end of the bolt 3 there is provided a threaded taperingcentric recess 10, in which is screwed in a threaded tapering portion 11of a connecting piece 12 having at its opposite end a cylindricalthreaded portion 13 and forming part of a hydraulic device intended forstretching the bolt 3 for creating therein a desired preload. Betweenthe unthreaded part 12 and the threaded part 13 of the connecting pieceis provided an adaptor 12a. This device incorporates a distance collar14 arranged to enclose the nut 9 while resting with its lower endsurface against the thrust washer 7. This distance collar 14 has aslotted opening 15 for allowing a tool 16 to reach the nut 9 fortightening it during assembly. The hydraulic device further incorporatestwo substantially annular members 17, 18 forming a hydraulic cylinder inwhich is arranged a hydraulic piston 19, rigidly connected to thethreaded end portion 13 of the connecting piece 12, and also the earliermentioned adaptor 12a, rigidly connected to the end portion 13 andserving as a second hydraulic piston. A connector 20 is provided in thepiston 19 and connected to a duct 21 for supply of hydraulic pressurefluid to the space between piston 19 and the hydraulic cylinder 18,whereas another connector 20a is provided in the wall of the annularmember 17 for supply of hydraulic pressure fluid to the space betweenthe adaptor 12a and the member 17. Both connectors are preferably fedwith pressure fluid of the same pressure simultaneously. Anotherconnector 22 is connected to a duct 23 communicating with the interiorof the hydraulic cylinder via a retraction piston 24 (see FIG. 3).

So far the device for causing preloading of the bolt is substantiallyearlier known. When a hydraulic fluid under pressure is introducedbetween the hydraulic cylinder 17, 18 and the hydraulic piston 19, thiswill move axially in upwards direction as shown in the drawing, therebysubjecting the bolt 3 to a tensile force stretching the bolt 3, whichcan then be secured in this preloaded state by tightening the nut 9, viathe tool 16. If however the bolt during such preloading should break,due to material failure or for other reasons, heavy parts of theequipment could come loose and be thrown upwards forcefully, if therewere no safety device to prevent such incidents from happening.

For this reason there now is provided a safety device incorporating anouter sleeve 25 arranged coaxially with and rigidly attached via screws26 (see FIG. 2) to the top end of the threaded end portion 13 of theconnecting piece 12. In the interior of the outer sleeve 25 and at theend thereof turned away from the connecting piece 12 there is arrangedan intermediate sleeve 27, wherein is provided an expanding plug 28,which has a central through-hole 29. This central through-hole 29 in theexpanding plug 28 is aligned with corresponding through-holes 30, 31 inthe connecting piece 12 and its tapering portion 11 and in the bolt 3.At the bottom of the bolt 3 in said through-hole 31 there is provided aninsert 32 with an internally threaded bore 33 (see FIG. 4). A tensilebar 34 is provided to extend through this series of aligned bores 29,30, 31, having its lower end firmly but detachably screwed into thethreads 33 of the insert 32, and being received in the expanding plug 28and being secured to the expanding plug 28 via a nut 35 provided aboveit.

If a bolt rupture should happen when this type of safety device is used,the centrally positioned tensile bar 34 in combination with the frictionbetween the expanding plug 28, the sleeve 27 and the cylinder 35, willascertain that the released energy will be absorbed without risking thatthe elements that may part from the bottom piece of the bolt during abreak will be thrown away.

By combining the arrangement described with a dial indicator 36contacting the top part of the tensile bar 34 it is furthermore possibleto measure the instant elongation and preloading of the bolt 3.

After stretching and tightening each bolt of a bolt connection in themanner described, the hydraulic device and the safety device shallpreferably be dismounted and this can easily be done by unscrewing thetensile bar 34 from its connection to the insert 32 at the bottom of thebolt 3, and the tapering part 11 of the connecting piece 12 from therecess 10 in the upper end of the bolt 3, whereupon the entire unit canbe lifted with use of a lifting bar 37 fitted to the hydraulic device.

The invention is not limited to the embodiment shown in the drawings anddescribed in connection thereto but modifications and variations arepossible within the scope of the attached claims.

What is claimed is:
 1. A safety device for use when hydraulicallysubjecting a bolt to a high preload in order to provide said bolt withan internal stress in a mounted condition, wherein a first end of saidbolt is secured to a first element and extends through a second elementwhich is clamped to said first element, and where a hydraulic preloadingdevice is temporarily applied to a second end of said bolt for axiallystretching the bolt to create said internal stress, the safety devicecomprises:a safety tensile bar detachably connected at one end thereofto said first end of the bolt and extending through a central bore inthe bolt and further through said hydraulic preloading device, anexpanding plug; and a sleeve, wherein said safety tensile bar being heldadjacent its opposite end in the expanding plug and the expanding plugis in frictional contact with the sleeve for guiding said plug andprovided to create a counterhold absorbing relieved energy in case ofthe breaking of the bolt during the preloading operation.
 2. A device asclaimed in claim 1, wherein the expanding plug and its guiding sleeveare detachably fitted to and resting upon the part of the pre-loadingdevice facing away from said bolt.
 3. A device as claimed in claim 1,wherein said top end portion of the tensile bar is arranged to engagethe measuring tip of a dial indicator for indicating the instant axialstretching of said bolt.